There are a couple different types of image sensor packages. Types of image sensor packages include CCD image sensor packages and CMOS image sensor packages. In the preferred embodiment, a full frame CCD sensor is studied and used for the purpose of explaining how to detect scratches, digs, and dirt. A full frame image sensor is basically a two dimensional array of pixel sensing elements. Full frame CCD image sensors capture light and store the resulting signal electrons in the individual pixel sensing elements. The pixels are vertically shifted down each column in parallel by one row, with the last row being shifted out and filling a horizontal shift register. These pixels in the horizontal shift register are then shifted out one at a time (serially) until the horizontal shift register is completely empty. At this time, the sensor is ready to fill the horizontal shift register again, and the process of parallel to series shift explained above is repeated one row at a time until all rows of the sensor have been transported out of the sensor. The output pixels are then reconstructed to form a digital image that can be processed and analyzed for scratches, digs, and dirt.
In typical image sensor packages, there is a transmissive surface that protects the image sensor and often has coatings on it for the purpose of cutting down on reflectance or blocking infrared radiation. If this transmissive surface contains blemishes that obstruct or distort light coming through it, then the resulting image is of a lesser quality. These blemishes are classified into three different types: dirt, which can be removed by cleaning; scratches, which are usually long and narrow; and digs, which are usually small and more circular in nature. These blemishes are characteristics of the transmissive surface that is attached to and covers the image sensor. The shadows cast by these blemishes appear on the sensor elements when the incoming light passes through the transmissive surface, and thus appear on the resulting image. Scratches, digs, and dirt are typically created during the handling and packaging in the manufacturing process of the image sensor packages, but are also created later during the assembly in the final product, such as a digital camera. A certain level of corruption in digital image would be acceptable. In the past, the detection of blemishes on the transmissive surface has been done under a microscope or by viewing an image of the transmissive surface on a screen. These techniques are subjective, time consuming, and inaccurate.
These blemishes are different from the corrupted data classified as pixels, columns, and defective cluster pixels as referenced in U.S. patent application Ser. No. 10/100,723 filed Mar. 19, 2002 by Timothy G. Wengender, et al, the disclosure of which is incorporated herein by reference. The way in which corrupted data is differentiated is through focusing or defocusing the lens aperture 51. To detect sensor defects such as pixels, columns, or clusters, the lens aperture 51 is defocused as much as possible by placing the aperture at it's minimum setting (a small f-number is a large diameter aperture). At this setting the light that passes through the transmissive surface is diffused and any scratches, digs, or dirt are blurred and become unnoticeable on the image sensor package. To detect scratches, digs, or dirt, the lens aperture 51 is focused by placing the aperture at or near it's maximum setting (a large f-number is a small diameter aperture). At this setting, the light that passes through the transmissive surface is not diffused and any scratches, digs, or dirt are focused and become noticeable on the image sensor.
Such scratches, digs, or dirt will cause corrupted data in the digital image after it is read out of the image sensor package. To produce the highest quality image, the scratches, digs, or dirt need to be identified during the manufacturing and assembly process so that image sensor packages can be accepted or rejected in the final assembly.